Post parameters are not being received by server

Question

I'm developing an android app that needs to interface with a server. To test that my HTTP post stuff is working, I am using posttestserver.com. Alas, while the messages I send are received, the Post parameters I tried to send along are not received. I have the internet permission enabled in my Manifest (something I saw come up a lot in other questions) but still, I can't get the data to go through. My code is:

public String uploadData(String dataLine) throws MalformedURLException {
    URL url = new URL(urlName);
    HttpURLConnection urlConnection;
    String text = "";
    BufferedReader reader = null;
    try {
        urlConnection = (HttpURLConnection) url.openConnection();
        urlConnection.setDoOutput(true);
        urlConnection.setDoInput(true);
        urlConnection.setChunkedStreamingMode(0);
        urlConnection.setRequestMethod("POST");
        urlConnection.setRequestProperty("Content-Type", "application/x-www-form-urlencoded");

        Uri.Builder builder = new Uri.Builder()
                .appendQueryParameter("key", key)
                .appendQueryParameter("data", dataLine);
        String query = builder.build().getEncodedQuery();

        OutputStream os = urlConnection.getOutputStream();
        BufferedWriter writer = new BufferedWriter(
                new OutputStreamWriter(os, "UTF-8"));
        writer.write(query);
        writer.flush();
        writer.close();
        os.close();
        urlConnection.connect();

        reader = new BufferedReader(new InputStreamReader(urlConnection.getInputStream()));
        StringBuilder sb = new StringBuilder();
        String line = null;

        // Read Server Response
        while((line = reader.readLine()) != null)
        {
            // Append server response in string
            sb.append(line + "\n");
        }
        reader.close();


        text = sb.toString();
    } catch (IOException e) {
        e.printStackTrace();
    }
    return text;

}

I was originally building the Post data with something like

String POST = "key="+URLEncoder.encode(key, "UTF-8");

but in a different thread, I saw this method. (How to add parameters to HttpURLConnection using POST)

http://posttestserver.com/data/2015/07/27/14.39.441876894412 this is one of the POST requests that didn't bring the parameters along.